This invention relates to a titania resistive-type gas sensor for detecting oxygen in automotive exhaust gases. More particularly, this invention relates to an oxygen sensor wherein electrical resistance is measured through a titania pellet between embedded very thin platinum wires, which sensor is ruggedly constructed for prolonged use on board an automotive vehicle.
The electrical resistivity of titanium dioxide, more commonly called titania, is known to change in response to changes in the ambient oxygen partial pressure. Accordingly, the oxygen content of automotive exhaust gases may be monitored by exposing a titania pellet to the gases and measuring its electrical resistance. A suitable titania pellet is manufactured by compacting and sintering titania particles, as described, for example, in U.S. Pat. No. 4,249,156, issued to Micheli in 1981. In order to obtain good electrical contact with the titania, very thin platinum wires are embedded in the compact and cofired with the titania to produce an intimate bond. The platinum wires are very thin, typically on the order of 0.008 inch diameter, to prevent damage to the pellet due to differential thermal expansion, either during firing or during automotive operations. The thin wires are connected to thicker terminals, which in turn are connected to a resistance measurement circuit. However, when the pellet is incorporated into a sensor and mounted in an exhaust system on board an automobile, vibrations, thermal cycling and other conditions associated with automotive operation tend to break the very thin wires or to loosen the wire-pellet connections, particularly where the pellet is suspended in the gas stream solely by the wires.
Therefore, it is an object of this invention to provide an improved resistive-type exhaust gas oxygen sensor having a durable construction for better withstanding automotive operating conditions to extend the useful life of the sensor on board a vehicle. The sensor comprises a titania pellet immovably wedged in position therein. Thus, the pellet is not supported by and does not stress any attached thin wires required for electrical connections. In addition, this invention provides a sensor that can be readily assembled so as to wedge the pellet in position.